1. Field of the Invention
The present invention relates to a variable valve system, and more particularly to a variable valve system for changing the operating angle and/or lift amount of an internal combustion engine's valve body.
2. Background Art
A conventionally known variable valve system, which is disclosed, for instance, by Japanese Patent Laid-Open No. 2002-349215, is capable of changing the lift amount of an internal combustion engine's valve body by changing the rotation position of a control shaft. This conventional system comprises a rotation angle sensor, which varies its output in accordance with the rotation position of the control shaft, and a rotary drive mechanism, which provides feedback control over the rotation position of the control shaft in accordance with the sensor output.
More specifically, the above conventional variable valve system performs a process for detecting the current rotation position in accordance with the difference between the reference output and actual output of the rotation angle sensor, and also performs a process for controlling the rotary drive mechanism to ensure that the detected current rotation position coincides with a target position. When these processes are performed, the control shaft rotation position agrees with the target position, thereby providing a target lift amount.
To obtain the target lift amount accurately by performing the above processes, it is necessary to ensure that the rotation angle sensor output accurately agrees with the actual rotation position. In general, however, the relationship between the sensor output and rotation position cannot be predefined due to varying rotation angle sensor mounting positions and various changes with time.
Under the above circumstances, the above conventional variable valve system includes a stopper for inhibiting the control shaft from making an unnecessary revolution. More specifically, the conventional variable valve system includes a stopper for restricting the rotation of the control shaft at a rotation position at which a minimum lift is generated. While the stopper provides restriction, the conventional variable valve system recognizes the rotation angle sensor output as the reference output. In this instance, it is assured that the reference output corresponds to the rotating end of a small lift side of the control shaft.
Being based on the reference output that assuredly corresponds to the rotating end of the control shaft as described above, the above conventional variable valve system acquires the actual output of the rotation angle sensor. Therefore, the mechanism employed by the above conventional variable valve system accurately detects the rotation position of the control shaft without being affected, for instance, by varying rotation angle sensor mounting positions.
Including the above-mentioned document, the applicant is aware of the following documents as a related art of the present invention.
[Patent Document 1]
Japanese Patent Laid-Open No. 2002-349215
[Patent Document 2]
Japanese Patent Laid-Open No. 2000-234507
However, the stopper of the above conventional variable valve system is positioned at an end point of a control shaft's normal rotation range. When this configuration is employed, it is likely that the control shaft will frequently have a mechanical collision with the stopper under normal operating conditions. Even if the rotary drive mechanism is controlled so that the control shaft stops rotating at the above-mentioned end point, the control shaft does not accurately stop rotating at the end point due, for instance, to the inertia of a rotation system and an error in rotation position detection. Therefore, when the above-mentioned conventional configuration is employed, the minimum lift is demanded so that the control shaft is likely to frequently collide against the stopper.
If the control shaft frequently collides against the stopper, the stopper and the member colliding against the stopper are likely to wear and deform. When such a collision occurs, the rotary drive mechanism is significantly impacted so that the components of the rotary drive mechanism may also wear and deform. Thus, it is likely that various components of the above conventional variable valve system will prematurely deteriorate.